This invention relates to a method of and apparatus for sorting radiation emissive material, and in particular it relates to a method of and apparatus for sorting radioactive particles of ore.
In the following description reference to the property of radioactivity is intended to include natural radioactivity such as is associated with uranium ore for example, and radioactivity induced by excitation with, for example, neutrons, gamma rays or x-rays. For convenience the description will pertain mainly to the sorting of ore particles containing U.sub.3 O.sub.8 but it is intended that the invention be directed to the sorting of any ore which has natural or induced radioactive properties.
In the sorting of radioactive ores, each piece or particle may contain a certain amount of radioactive material such as U.sub.3 O.sub.8. In other words each particle has a definite grade or assay value, and a representative sample of pieces will exhibit a range of grades typical of the value distribution of the particular ore deposit. Knowing the price of uranium, the cost of further milling, and other secondary factors, a "cut-off" grade may be established which represents a lower limit of profitability at that point in the milling process. Particles below this cut-off grade may be profitably discarded. This is the economic basis of sorting. It is, of course, desirable to discard particles below cut-off grade early in the milling process.
The cut-off grade is a ratio or percentage, that is it is an absolute value of U.sub.3 O.sub.8 related to mass. All cut-off grade particles will have absolute values of contained U.sub.3 O.sub.8 related to mass and gamma activity is related to the absolute value of U.sub.3 O.sub.8. For example, ignoring self-shielding within the rock, detector geometry and other secondary factors, the detected radioactivity or "gamma count rate" from 1 inch, 2 inch and 3 inch cubes of identical grade material would be approximately in the proportion of 1:8:27. Therefore it is important to take mass or size into consideration as well as the gamma count rate. This has been done in the past (a) by screening the particles to have them within a certain size range, (b) by measuring the mass such as by weighing, or (c) by determining mass from a size measurement such as might be found by scanning the material to obtain a projected area either in one plane or two different planes and using the scanned areas to estimate mass.
Canadian Pat. No. 467 482 to Lapointe, issued Aug. 22, 1950 describes an apparatus for sorting ore particles where the particles are sized and then proceed in single line arrangement past the radiation detector. This is an example of sorting apparatus referred to in the preceding paragraph under (a). The suggested speed of the particles for a size range of 8 to 15 mm diameter is about 3 to 10 m per minute. This is a relatively slow speed. Furthermore, this broad size range would not give accurate results compared to individually ascertaining the particle size.
U.S. Pat. No. 3,052,353 to Pritchett, issued Sept. 4, 1962, describes an ore sorting device which may determine the mass of each ore particle, as referred to in (b) above, by passing the ore over a form of weighing device. This patent also describes means for determining mass from a projected area as referred to in (c) above. The ore particles move in a single line, one by one, past a radiation detector.
In the prior art sorting devices it is necessary to have each particle in the immediate vicinity of a radiation detector for a sufficient length of time to obtain a reliable "count" (i.e. a measurement of radiation). A radiation detector, for example a scintillation counter for gamma detection, may be gated on for a predetermined fixed period of time as each particle is immediately adjacent during its passage past the scintillation counter. The fixed period of time is usually related to rock length and the speed of the particle past it. However, because radiation is a random phenomenon, the count may not be representative if the fixed period of the gate is short. Consequently it has been the practice to obtain a more representative count and a more accurate measurement of radiation, by having a longer period when the scintillation counter is gated on. This means the particles must move slowly. In addition, for the same detector arrangement, it takes longer to assess a small particle than a large particle. This is because the radiation will be less and the count will consequently accumulate more slowly. The rate of movement of the particles must be related to the determination of the "cut-off" count for the smallest particles being sorted. This has a drastic effect on throughput and has limited the commercial application of radiometric sorting apparatus.
It should be noted that sorting of most uranium ores may not be an economic proposition if the sorting apparatus can handle only larger size ranges. Furthermore discarding of large particles may discard too great an amount of useful ore. If it were broken into smaller particles, many might be above cut-off grade and be economically processed.
Thus, while it is desirable to sort radioactive ore particles of smaller sizes, it is difficult because it takes longer to accumulate a count of significance, and consequently slows the sorting rate. In addition it is difficult to control background radiation in a uranium mill environment and with smaller particles the count becomes closer to the background count.
Attempts have been made to overcome or reduce the difficulties of sorting small particles or radioactive ore. These attempts generally fall into three groups as follows:
1. Increasing detector size. PA1 2. Using opposed detectors. PA1 3. Using multiple detectors. PA1 (a) moving the particles to be sorted, one at a time, into a predetermined position adjacent a radiation detector, and temporarily retaining the particle in that position for a time period yet to be determined, PA1 (b) deriving a first signal from said radiation detector representing radiation from the particle and accumulating it with respect to time, PA1 (c) comparing the accumulating first signal with values representing a cut-off rate of radiation and providing a second signal when the first signal exceeds the values by a first predetermined amount and a third signal when the first signal is less than the values by a predetermined amount, and PA1 (d) moving the particle from its temporary position along a first path as soon as a second signal is provided and along a second path as soon as said third signal is provided.
It is perhaps self-evident that an increase in the size of the detector will accumulate a count more rapidly and consequently permit a faster throughput. There is, however, a limit to the size that is effective. For example, there is an optimum crystal size and geometry for a scintillation detector for a given particle size and increasing size beyond this produces diminishing returns on the count rate, but background count increases in proportion to crystal volume. In addition, interference from radiation of adjacent particles in the line becomes more of a problem so more space must be left between pieces. The cost of crystals also increases very rapidly with volume.
The use of opposing detectors can significantly increase count rate if the particles are closely sized. However, the general run of particles is frequently found with varying heights and widths and the opposing detectors must be separated by a sufficient distance to avoid jams. Because of the varying distance from at least one detector, there may be a variable introduced. If, however, the particles are closely sized the use of opposing detectors is satisfactory.
Multiple radiation detectors are another arrangement that has been tried, and it permits a faster particle speed and increased throughput. Several detectors are placed in series and the count for a particle is detected as the particle passes each detector and the counts are placed in an accumulator. This is in effect the equivalent of slower movement past a single detector. U.S. Pat. No. 2,717,693 to Holmes, issued Sept. 13, 1955 describes such a multiple detector system. While the use of a multiple detector arrangement increases permissible particle speed, it has on the other hand some disadvantages. For example, shielding and particle separation are more difficult to achieve in a fast feed, series detector configuration. Scintillation detectors in series must be matched or compensated and failure of one will affect the whole series. As with any constant feed rate system, counts are accumulated while the particle approaches and then leaves the optimum detection position, the count is decreased but the background count is not, and hence the ratio of count to background is degraded. Also, as speed increases rocks or particles are more difficult to control and rolling will cause invalid results. In summary, it has been said that six separate slow feed sorters with single detectors may give better results than a fast feed sorter with six series detectors, and breakdowns will be less critical.